1. Field of the Invention
This invention relates to needles for use in performing lumbar puncture procedures and, more particularly, to a novel, atraumatic needle apparatus and method for significantly reducing postdural puncture headaches.
2. The Prior Art
Spinal anesthesia is one of the most frequently employed methods of regional anesthesia. This regional anesthesia is accomplished by the temporary interruption of nerve transmission using a local anesthetic injected into the readily identifiable subarachnoid space. The ensuing anesthesia is predictable, occurs rapidly, and is associated with profound muscle relaxation. The patient may be wide awake, or if preferred, the anesthetic may be supplemented with varying amounts of sedative-tranquilizers. Spinal anesthesia is particularly useful for surgery involving the lower extremities, pelvis, perineum, and lower abdomen.
The spinal column, which surrounds the spinal cord, is formed by a series of vertebrae separated by cartilaginous intervertebral disks and united by a series of ligaments. The body of each vertebra bears the weight of the patient and forms the base of the neural arch. The arch, which surrounds the spinal cord, is made up of a pedicle and lamina on each side. Between the laminae of each vertebra there is a posterior opening in the vertebral canal. It is through this opening that a spinal needle is passed when performing a subarachnoid block.
In adults the spinal cord varies in length from 40 to 45 cm. and ends at various levels of the vertebral column depending on the age of the patient. In the newborn, the spinal cord extends to the third lumbar vertebra, but in the adult it usually ends at the lower border of the first lumbar vertebra because the spinal cord does not grow as much as the vertebral column. Thirty-one pairs of symmetrically arranged spinal nerves are each attached to the spinal cord by an anterior and posterior root. Because the spinal cord is shorter than the vertebral column, the spinal cord segments in adults do not lie opposite their corresponding vertebrae. The spinal nerve roots must travel obliquely in a caudad direction to reach their respective intervertebral foramina. The roots of the lumbar, sacral, and coccygeal nerves comprise the cauda equina and are necessarily the largest and longest in order to reach their intervertebral foramen. The greater size of these nerve roots provides a larger surface area to be exposed to the action of local anesthetics, thus allowing more rapid onset of anesthesia.
The spinal cord is covered by three membranes or meninges. The dura mater (the outermost membrane) is the downward continuation of the meningeal layer of the cranial dura mater. The middle of the three coverings, the arachnoid is a thin membrane closely adherent to the dura mater. The dura and the arachnoid are in such close contact that usually it is not possible to puncture the dura without also piercing the arachnoid. Nevertheless, on rare occasions, the tip of the conventional epidural or spinal needle may accidentally enter the subdural space. Local anesthetic inadvertently injected into the subdural space will diffuse poorly and result in inadequate contact with the nerve roots. Poor or absent anesthesia may ensue. Should subdural placement occur during an attempted epidural anesthetic, the improper position of the needle may not be recognized and the injection of an epidural dose of local anesthetic may result in a much higher block than anticipated.
The innermost membrane, the pia mater, is a thin, delicate, highly vascular membrane closely adherent to the spinal cord. The space surrounding the pia is filled with cerebrospinal fluid and is enclosed externally by the arachnoid. In addition to spinal fluid, this space contains the spinal nerve roots and the main blood vessels of the central nervous system. In the cervical and thoracic regions, the space is only about 3 mm deep, but below the lower border of the first lumbar vertebra, where the spinal cord usually ends, the space has a diameter of about 14 to 15 mm.
A spinal needle 9 cm long is usually adequate, but longer ones (10-15 cm) are available for the occasional obese patient or difficult paramedian approach. A removable, close-fitting stylet helps stiffen the needle and prevents coring of the tissue. Commonly, two sizes of spinal needles are used, 22 gauge and 25 gauge. The larger diameter 22 gauge needle is easier to direct and renders the characteristic feel of the various ligaments penetrated easier to appreciate. However, the incidence of postspinal headache is increased with the larger needle, particularly if the larger needle is also equipped with a standard point which is a cutting bevel.
A postdural puncture headache is the most common postoperative complication of spinal anesthesia. The incidence increases with the larger spinal needles and those with a cutting bevel at the tip but decreases with increasing patient age. Postdural puncture headache also occurs more commonly in women than in men, and more often in pregnant women than in nonpregnant women. The headache is positional in that it comes on in the upright position and is relieved or at least improved in the recumbent position.
The causative mechanism of the postdural puncture headache is believed to be associated with the continuing leakage of cerebrospinal fluid (CSF) through the dural opening left by the spinal needle. The leakage of CSF causes a decrease in CSF pressure which, in turn, produces compensatory cerebral vasodilation. Bringing the patient into the erect position also results in traction on the pain-sensitive, dilated blood vessels. Accordingly, conservative therapy for the postdural puncture headache consists of bed rest and analgesics.
Various preventive measures for the postdural puncture headache have been advocated. The common practice of keeping the patient supine for 4 to 24 hours after lumbar puncture has been shown to be ineffective. For a standard point needle having a cutting bevel at its tip, insertion of the needle with the bevel parallel to the longitudinal fibers of the dura appears to produce a smaller rent in the dura with a lower incidence of headache. Pencil point needles such as the commercially available Whitacre and Sprotte needles also have a lower incidence of headaches. These pencil-point needles have a closed pencil point created when the open end of the needle is swaged closed, as the name implies, like a pencil point or, more accurately, with a conical apex. This conical apex is believed to spread, rather than cut, the predominately longitudinal dural fibers and, on removal of the needle, the resulting dural hole should be smaller and seal off more rapidly. Indeed, studies have shown that the incidence of postspinal headache when a 22 gauge conical apex needle is used is comparable to that following use of the much smaller 26 gauge, bevel needle.
In an attempt to suitably occlude the dural opening to minimize leakage of CSF, an available procedure is to create what is known as a blood patch. This is done by obtaining 10 to 20 cc of blood from the patient and injecting this volume of blood into the tissue adjacent the puncture site of the spinal needle. This relatively large volume of blood is required since it is virtually impossible for the health care professional to exactly position the blood patch directly over the original puncture site. In effect, therefore, the blood patch is designed to seal the dural puncture thereby significantly minimizing the frequency of the postdural headache.
However, even with these improvements, postdural puncture headache remains a problem although the frequency is significantly reduced. Further, an adequate flow rate of anesthetic through the smaller needle is also a concern particularly with the pencil-point or conical-apex needle since the injection is through a side port located an incremental distance behind the tip. The placement and size of this side port is an important feature since it affects both the distribution of the anesthetic as well as overall strength of the needle. One particular needle (Sprotte, see FIG. 1, Prior Art) has a relatively long side port which has been found in certain circumstances to deliver anesthetic to both sides of the dura since the length of the side port is greater than the thickness of the dura. Another prior art needle (Whitacre) has a shorter side port but is cut wider across the needle to overcome this shortcoming. A wider side port, however, weakens the needle particularly if the side port is any appreciable distance from the pencil-point tip.
Another disadvantage to the presently available pencil-point-tip spinal needles is that the sides of the tip are generally straight in a true cone configuration. Thus, a relatively abrupt shoulder is formed as a ridge at the juncture between the sloped sides of the conical tip and the cylindrical side walls of the body of the needle. It is currently postulated that this relatively abrupt change in the profile of the needle excessively distorts the dura and thereby contributes to the presence of a post puncture hole in the dura.
In view of the foregoing, it would be an advancement in the art to provide a spinal needle having a pencil-like point with a gently rounded profile to reduce the trauma to the fibers of the dura. It would also be an advancement in the art to provide a spinal needle of which the side port has an opening with a cross sectional area only incrementally larger than the cross sectional area of the lumen of the hollow needle. It would also be an advancement in the art to provide a spinal needle having a side port immediately adjacent the pencil-like point thereby reducing the length of the moment arm between the tip of the pencil-like point and the midline of the side port. Such a novel spinal needle is disclosed and claimed herein.